In modern exhaust gas turbochargers, to increase the intake pressure of internal combustion engines, single-stage radial compressors with bladed diffusors at the compressor impeller outlet are generally used. The diffusors have a guide device with guide blades with prismatic, generally aerodynamic profiles. The outflow of the compressor impeller is hallmarked essentially by a so-called jet wake flow in which radially low-momentum flow collects on the side of the inlet housing (shroud side), while on the hub side there is a healthy flow. Thus the radial velocity component of the flow drops over the height of the diffusor channel.
In the design of compressor stages a compromise must always be found between the aerodynamic performance, mechanical load and acoustic noise formation by a compressor. Compressor stages with high specific absorption capacity have long moving blades with natural vibration forms which occur at relatively low frequencies and which are very easily excited and caused to oscillate. The main source of these excitations is a nonuniform pressure potential field which is produced by the guide blades of the guide device of the diffusor. According to experience, the compressor stages with high specific absorption capacities also produce high noise levels at the compressor outlet which are caused essentially by the impact of the flow fluctuations over time on the guide blades of the guide device of the diffusor.
One focus of continuing research for improving the stage consists in designing radial compressors such that the pump limit is shifted to flow rates as small as possible without in doing so having to tolerate adverse effects on efficiency. If the characteristic diagrams are narrow, numerous specifications with different blade heights and/or diffusors for matching to the varied application must be observed; this leads to high storage costs.
To reduce blade vibrations the moving blades of the compressor impeller can be thickened or the distance of the guide device of the diffusor to the compressor impeller outlet can be increased to reduce excitation. The increase of the distance of the guide device of the diffusor leads at the same time to a noise reduction. But both measures lead to a reduction in efficiency, for which reason the desired thermodynamic performance would be missed.
In order to correspond to the described flow lamination, guide blades tilted meridionally can be used in the guide device of the diffusors. This results in that faulty incident flow on the guide blade incidence edges is minimized and thus better efficiency is achieved. In addition, the meridionally tilted guide blades cause a reduction of the vibration excitation of the guide blades and reduced exit noise generation by the blade wheel/guide wheel interaction. When using meridionally tilted guide blades in the guide device of the diffusors at high absorption capacities strict acoustic requirements cannot be met under certain circumstances. Production of diffusors with meridionally tilted guide blades is moreover much more complex than that of prismatic blades. Instead of being able to cut a blade in one cutting process, the profile must be shaved over the height in several steps.
U.S. Pat. No. 5,178,516 discloses a guide device on the compressor impeller outlet of radial compressors in which to improve flow properties the inlet edges of the guide blades on the inlet side are nearer the compressor impeller outlet than on the hub side. With the inlet-side lengthening of the guide blades, the result should be that no recirculation flows occur locally directly radially outside the compressor impeller outlet. Here it is explicitly stipulated that the maximum distance of the blade inlet edge on the entry side should be less than 10 out of one hundred of the outside radius of the compressor moving blades. The set-back inlet edge on the hub side is accordingly spaced more than 10 out of one hundred of the outside radius of the compressor moving blades away from the compressor impeller outlet. The height of the lengthened blade tip is stipulated at 10 to 60 out of one hundred of the height of the diffusor channel.
U.S. Pat. No. 5,529,457 discloses from a similar arrangement that for simpler production different inlet edges of the guide blades can be implemented by one stage.
With the lengthening of the guide blade tips which was proposed in these two documents the attempt is made to enlarge the operating region of the compressor. Noise abatement measures play no part in the details. Due to the reduction of the distance of the inlet edges of the guide blades from the compressor impeller outlet, with the suggested stepping of the inlet edge of the guide blades the noise at the compressor outlet and excitation of blade vibrations however should rather increase.